From ea85b81ccdb28dda636351e80a1888b1d279f325 Mon Sep 17 00:00:00 2001
From: sunmy2019 <59365878+sunmy2019@users.noreply.github.com>
Date: Sun, 22 Mar 2026 08:10:17 +0800
Subject: [PATCH] Support fmt::runtime with wchar_t in fmt::format_to_n (#4715)

---
 include/fmt/xchar.h | 12 ++++++++++--
 test/xchar-test.cc  | 20 ++++++++++++++++++++
 2 files changed, 30 insertions(+), 2 deletions(-)

diff --git a/include/fmt/xchar.h b/include/fmt/xchar.h
index 31706f96..6d95a737 100644
--- a/include/fmt/xchar.h
+++ b/include/fmt/xchar.h
@@ -279,10 +279,18 @@ inline auto vformat_to_n(OutputIt out, size_t n, basic_string_view<Char> fmt,
   return {buf.out(), buf.count()};
 }
 
+template <typename OutputIt, typename... T,
+          FMT_ENABLE_IF(detail::is_output_iterator<OutputIt, wchar_t>::value)>
+FMT_INLINE auto format_to_n(OutputIt out, size_t n, wformat_string<T...> fmt,
+                            T&&... args) -> format_to_n_result<OutputIt> {
+  return vformat_to_n(out, n, fmt.get(), fmt::make_wformat_args(args...));
+}
+
 template <typename OutputIt, typename S, typename... T,
           typename Char = detail::format_string_char_t<S>,
-          FMT_ENABLE_IF(detail::is_output_iterator<OutputIt, Char>::value&&
-                            detail::is_exotic_char<Char>::value)>
+          FMT_ENABLE_IF(detail::is_output_iterator<OutputIt, Char>::value &&
+                        !std::is_same<Char, char>::value &&
+                        !std::is_same<Char, wchar_t>::value)>
 inline auto format_to_n(OutputIt out, size_t n, const S& fmt, T&&... args)
     -> format_to_n_result<OutputIt> {
   return vformat_to_n(out, n, fmt::basic_string_view<Char>(fmt),
diff --git a/test/xchar-test.cc b/test/xchar-test.cc
index cfe8389d..f6620ddf 100644
--- a/test/xchar-test.cc
+++ b/test/xchar-test.cc
@@ -146,6 +146,26 @@ TEST(format_test, wide_format_to_n) {
   EXPECT_EQ(L"BC x", fmt::wstring_view(buffer, 4));
 }
 
+TEST(format_test, wide_format_to_n_runtime) {
+  wchar_t buffer[4];
+  buffer[3] = L'x';
+  auto result = fmt::format_to_n(buffer, 3, fmt::runtime(L"{}"), 12345);
+  EXPECT_EQ(5u, result.size);
+  EXPECT_EQ(buffer + 3, result.out);
+  EXPECT_EQ(L"123x", fmt::wstring_view(buffer, 4));
+  buffer[0] = L'x';
+  buffer[1] = L'x';
+  buffer[2] = L'x';
+  result = fmt::format_to_n(buffer, 3, fmt::runtime(L"{}"), L'A');
+  EXPECT_EQ(1u, result.size);
+  EXPECT_EQ(buffer + 1, result.out);
+  EXPECT_EQ(L"Axxx", fmt::wstring_view(buffer, 4));
+  result = fmt::format_to_n(buffer, 3, fmt::runtime(L"{}{} "), L'B', L'C');
+  EXPECT_EQ(3u, result.size);
+  EXPECT_EQ(buffer + 3, result.out);
+  EXPECT_EQ(L"BC x", fmt::wstring_view(buffer, 4));
+}
+
 TEST(xchar_test, named_arg_udl) {
   using namespace fmt::literals;
   auto udl_a =